Name:   Lifer - Subject:   Here is what I learned the hard way Date:   12/14/2016 10:43:56 AM

If you are going to staple or nail the plastic under the joists you must make sure that all HVAC vents/piping is properly sealed to prevent any leakage. If not you have warm and cool air mixing which causes condensation which leads to pooling of moisture which causes way more problems than the vapor barrier alleviates. Also all holes where pipes pass through the floor must be air tight for the same reason. I grew up on a slab house and when I bought my first house it had a crawl space so I insulated with the plastic barrier.  I wished I had left the plastic out of the equation a few years later when I was replacing insulation. The was before mold became homeowner enemy number one too. Looking back I probably should be dead from lung disease. In this zone  the usefulness of a vapor barrier is debatable IMHO especially on older homes crawl spaces. Some recommend simply covering any exposed dirt with plastic on the ground as opposed to hanging it also.

If you hang it make sure it is airtight and inspect it regularly.